lsp: Show better info on IdentLiteral hover

Instead of showing the documentation for the expected type, show the
signature of the referenced object.

blueprintcompiler/language/gtk_menu.py

@@ -35,6 +35,13 @@ class Menu(AstNode):
     def id(self) -> str:
         return self.tokens["id"]
 
+    @property
+    def signature(self) -> str:
+        if self.id:
+            return f"Gio.Menu {self.id}"
+        else:
+            return "Gio.Menu"
+
     @property
     def tag(self) -> str:
         return self.tokens["tag"]